System and method for data scrambling to reduce the crest factor in an OFDM waveform

ABSTRACT

A method is provided for minimizing the peak power level in a combination of orthogonal frequency division multiplexed (OFDM) tones. The method is particularly effective when the information to be communicated is digital data consisting of long patterns of “1”s or “0”s. The digital data of the message is combined with a scrambling vector before the data is modulated. The randomization of data patterns, in turn, leads to a more random combination of OFDM tone amplitude being transmitted. The message is recovered at the receiver by removing the scrambling vector. A table of scrambling vectors is maintained at the transmitters and receivers. The table pointer to select the scrambling vector is selected in response to the position of the message in the communication structure. An OFDM communication system using the above-described method is also provided.

RELATED APPLICATION

This application is a continuation of prior U.S. patent application Ser.No. 09/590,004 filed on Jun. 8, 2000, now U.S. Pat. No. 6,785,258 whichclaims benefit of provisional application No. 60/140,648 filed on Jun.23, 1999.

This application claims the benefit of U.S. Provisional Application No.60/140,648, filed Jun. 23, 1999 and entitled “Method for High Speed DataCommunications in a Personal Wireless Access Network” which isincorporated herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates generally to communication systems and, moreparticularly, to a technique which minimizes the transmitted peak powerlevels in an orthogonal frequency division multiplexed (OFDM) waveformby randomizing the digital data being communicated.

The frequency tones transmitted in an OFDM system are purposely spacedso that the frequency components of the tones are non-additive. That is,multiple tones can be received in a wideband receiver without a firsttone canceling out or adding to the amplitude of neighboring tones. Thispermits the communication system to transmit and receive tonessimultaneously, without mutual interference. However, these orthogonalfrequency components still interact (are additive) with each other fromthe perspective of time domain analysis.

In the exemplary OFDM system described in Provisional Application Ser.No. 60/140,648, many orthogonal frequency tones are transmitted orreceived simultaneously. These tone frequency sums are ultimatelycommunicated by base stations and remote units through a wired or radiofrequency (RF) link using power amplifiers and receivers. At any oneinstant, the waveform being transmitted by a power amplifier is avoltage which is a sum of the voltages of the tones. If the phaserelationship between tones is random, then the peak voltage falls withina well defined range. For example, with 288 tones of equal magnitude,but random phases, the peak voltage in the time domain OFDM waveform isdistributed in the range of 8 dB to 14 dB above the RMS voltage level,but may be as high as 27 dB above it when the phases align. This largepeak voltage is known as “cresting” of the OFDM waveform.

The OFDM system requires a high degree of linearity in the communicationof the OFDM tones. Non-linear transmission or reception of a firstsignal generates harmonic and spurious products which have a frequencyrelationship to the first signal. In a system that depends on frequencyorthogonality, the non-linear communication of tones will generate toneproducts which act to destroy this relationship of orthogonality.Alternately stated, in an OFDM system which relies upon the amplitudecomponents of the tones to relay information, it is critical that theamplitude information is not degraded in the amplification process.Thus, in an OFDM system it is necessary to use linear power amplifiersto transmit the OFDM waveform.

As is well known in the art, the bias on these types of amplifiers(Class A) must be set especially high. Class A amplifiers draw a greatdeal of current regardless of the RMS value of the actual power beingtransmitted. The amount of power drawn by a Class A amplifier isproportional to the peak voltage that the amplifier must be capable ofamplifying. It is therefore desirable to keep expected peak waveformvoltage to a minimum. This permits a linear amplifier with a minimumdynamic range to be used, without clipping the OFDM waveform and thuscausing non-linearities. This also minimizes the power consumption ofthe hardware, which extends the life of the equipment, and in the caseof battery powered devices, such as remote units, extends the life ofthe batteries.

These same problems are a concern in the design of the receiver.Although receivers do not generally consume a great of deal power, ascompared to the power amplifier, dynamic range is critical. The receivermust have a dynamic range large enough to recover a transmitted signalat a large range of input power levels, where the strength of thereceived signal corresponds to the distance between the receiver andtransmitter.

In the OFDM system, digital data is modulated using QAM, QPSK, PSK, orother schemes, as are well known in the art. These modulation systemsrely on the amplitude and/or phase of a symbol to convey information.Ultimately, the phase and/or amplitude of the OFDM tone is used forcommunication of information. In voice communications, and in many datacommunications, the information and, therefore, the patterns of digitaldata representing the information are random. The random pattern ofdigital data translates through the modulation process into a randomselection of tone phases and amplitudes. When this random collection ofOFDM tone frequencies are transmitted simultaneously, it isstatistically unlikely that they will add to create a very large peak inthe time domain OFDM signal. In this situation, there is no need to beconcerned with saturating an OFDM power amplifier designed to operate inthis scenario. Saturation of the power amplifier due to a large peakvoltage is more likely to occur when the digital data to be communicatedis highly correlated or the same, for example, when information is beingsent that represents an image with a highly uniform background. Then,the data consists of a long pattern of “0”s or “1”s. In thiscircumstance, where the digital data forms a repetitious pattern, it islikely that the OFDM tones will have a highly correlated phase andamplitude relationship. This correlation of the phase leads to a largepeak in the OFDM waveform. In order to handle this large peak withoutclipping the OFDM waveform, the power amplifier must operate in a largerdynamic range than the range resulting from a random combination of toneamplitude and phases. Operation in a large dynamic range results in thepower amplifier consuming more power.

It would be advantageous if the peak power levels required fortransmission in an OFDM communication system could be kept to a minimumto preserve linearity, reduce power consumption, and simplify the designof the transmitter and receiver.

It would be advantageous if the dynamic range of the power levelsrequired for transmission in an OFDM communication system could beminimized to avoid clipping in the power amplifier and to reduce powerconsumption.

SUMMARY OF THE INVENTION

Accordingly, a method for minimizing the peak voltage of the tones in anOFDM communication is provided. The method comprises a transmitteraccepting a message including digital data; randomizing digital datapatterns in the message, and so creating a randomized message; mappingthe randomized message into OFDM tones; and transmitting the OFDM tones.Likewise, the method further comprises a receiver which essentiallyreverses the process: receiving the transmitted OFDM tones; de-mappingthe received OFDM tones into the randomized message; de-randomizing thepattern of digital data in the message; and receiving the message.

The randomizing or de-randomizing of the message includes combining thedigital data with a predetermined scrambling vector. The scramblingvector is a pseudo-random arrangement of digital data, which is eithercombined or removed from the digital data by using an exclusive- orprocess.

A plurality of scrambling vectors are provided, and the method furthercomprises selecting a first scrambling vector; using the firstscrambling vector, randomizing the pattern of digital data in themessage; and using the first scrambling vector, de-randomizing thepattern of digital data in the message.

A system for the communication of orthogonal frequency divisionmultiplexed (OFDM) tones having a minimized peak power level is alsoprovided. The system comprises a base station and at least one remoteunit. Both the base stations and remote units have transmitters toaccept a message including digital data. Each transmitter includes ascrambler to randomize the pattern of digital data in the message, amodulator to modulate the randomized message, and a mapper to map themodulated message into OFDM tones for transmission by a power amplifier.Both the base stations and remote units have receivers to accepttransmitted OFDM tones. Each receiver includes a de-mapper to de-mapreceived OFDM tones into a modulated message, a demodulator todemodulate the modulated message into a randomized message, and adescrambler to de-randomize the message.

The base station and remote unit transmitters include a table withscrambling vectors organized in rows. The transmitter scramblers employa selection algorithm to direct a pointer to a row in the scramblingvector table. Likewise, the communicating receiver in the base stationor remote unit includes a descrambler with the same table, using thesame selection algorithm to direct a pointer to the same row in thetable.

The transmitters transmit the message of OFDM tones with respect to asystem frame structure. Thus, the transmitted OFDM tones can be said tooccupy a first position in a frame structure. The transmitter scrambleruses this known first position information to select the specificscrambling vector to randomize the data in the message. Likewise, thereceivers receive OFDM tones in the first position of the framestructure, and the descrambler selection of the scrambling vector isresponsive to the first position of the OFDM tones in the framestructure.

The scrambling vector tables are organized into rows of scramblingvectors, where each row of the table represents a scrambling vector of16 bits. After the first 16 bits of the message, when the scramblingvector has reached the last bit in the table row, the pointer isincremented to the next row in the table, and the de/scrambling processcontinues until the end of the message.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the present inventive system forthe communication of OFDM tones with minimized transmitted peak powerlevels.

FIG. 2 illustrates the randomization process which scrambles digitaldata patterns in a message.

FIG. 3 illustrates a table of scrambling vectors that are used by boththe scrambler of the transmitter, and the descrambler of the receiver(see FIG. 1).

FIG. 4 is a 16-QAM constellation for encoding data using Gray Coding(prior art).

FIG. 5 illustrates an OFDM time domain waveform of 288 tones with noscrambling.

FIG. 6 illustrates an OFDM time domain waveform of 288 tones withscrambling.

FIG. 7 is a flowchart illustrating a method for transmitting an OFDMwaveform at a minimized peak power level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The specific data randomization process presented in the invention hasbeen tailored for use with the OFDM communication system described inprovisional application No. 60/140,648, filed Jun. 23, 1999 and entitled“Method for High Speed Data Communications in a Personal Wireless AccessNetwork”. Highlights of the OFDM system will be represented below tohelp illustrate the present invention. However, it should be understoodthat these concepts are applicable to other systems which rely on randomdata streams for maximum efficiency.

FIG. 1 is a schematic block diagram of the present inventive system forthe communication of OFDM tones with minimized transmitted peak powerlevels. The system 10 comprises at least one base station and at leastone remote unit. Both the base stations and the remote units includetransmitter and receiver sections. The transmitters and receiversections of the base stations are essentially identical for the purposeof this invention. Practically, because of FCC peak power limitations,the remote units are more likely to transmit fewer OFDM tonessimultaneously. However, for all intents and purposes, the descriptionof the transmitter applies equally well to base station and remote unittransmitters, and the description of the receiver applies equally wellto base station and remote unit receivers.

Transmitter 12 has a port which accepts a message on line 14 to achannel input. The message includes digital data which represents eithera voice communication or digitally formatted information. One practicalapplication for the present invention's data randomization process isfor the transmission of large messages at high data rates. This form ofdata is more likely to include significant patterns of repetitious data.

Transmitter 12 includes a scrambler 16 to accept the message. As isexplained in more detail below, scrambler 16 has an output on line 18 toprovide a randomized pattern of digital data, or a randomized message,also called a scrambled message. Modulator 20 accepts the scrambledmessage and provides a modulated form of the scrambled message on line22. QAM (quadrature amplitude modulation), PSK (phase-shift keying), andQPSK (quadrature PSK) modulation formats are particularly applicable;however, the invention is not limited to any specific modulation format.The modulated message is accepted by a mapper 24, which maps themodulated data into OFDM tones. It is at this stage of the process thatthe tone amplitudes and phases are assigned.

The exemplary OFDM system transmitter is capable of transmitting 288tones simultaneously. There are 16 frequency bands, with 18 tones ineach frequency band. However, the transmitter channel into which themessage has been inserted may use any number of frequency bands at anyone time for the transmission of the message. The remaining frequencybands are represented by lines 26 a, 26 b, and 26 n. The sum of thetones in all the frequency bands are represented by line 28, which isconnected to antenna 30. However, it should be understood that thesuccess of the present invention is not limited by the number offrequency bands assigned to a channel, or how many time slots a messageis spread across.

Receiver 50, whether it is a remote unit or base station receiver,operates to essentially reverse the modulation and scrambling to recoverthe message. Receiver 50 has an antenna 52 to accept the radiated signalfrom the transmitter 12. This signal on line 54 may carry OFDM tonesrelating to other channels connected on lines 56 a, 56 b, and 56 n,where the channels on lines 26 a and 56 a, 26 b and 56 b, and 26 n and56 n correspond, respectively. The message-bearing channel is acceptedby de-mapper 58, which converts the amplitude and phase of the tonesinto a modulated digital signal on line 60. It is understood that thede-mapper 58 also includes a tone equalizer function. The demodulator 62demodulates the message, using the same format as used by transmittermodulator 20. Then, the message is descrambled, or de-randomized, bydescrambler 66, and the original message is output on line 68.

FIG. 2 illustrates the randomization process which scrambles digitaldata patterns in a message. To create a message with a random pattern ofdigital data, the message (with a non-random pattern of digital data,such as a series of “0”s or a series of “1”s) is combined with ascrambling vector. Many processes would be applicable, but the presentinvention is illustrated using an exclusive-or addition process, alsoreferred to as modulo two addition. Every bit in the scrambling vectoris added to a corresponding bit in the message, and the carries in theaddition process are discarded. The result is most likely to be amessage with a random pattern of digital data. As shown, a messageincluding a series of “0”s is combined with a first scrambling vector.To recover the actual message from the randomized message, the firstscrambling vector is removed from the randomized message. That is, thefirst scrambling vector is removed by another exclusive-or additionprocess to recover the original message. Note that the same vector mustbe used to unscramble as was used to originally scramble the message.

The present invention is not limited to any particular length ofscrambling vector, or any particular number of scrambling vectors. Inthe exemplary OFDM system, the length of the scrambling vector has beenchosen to have a length of 16 bits, as shown in FIG. 2. Most messages,however, are longer than 16 bits. To prevent the emergence of digitaldata patterns which may result from the continual reuse of the samescrambling vector, a plurality of scrambling vectors are used for thescrambling of digital data.

FIG. 3 illustrates a table of scrambling vectors that are used by boththe scrambler 16 of the transmitter 12 and the descrambler 66 of thereceiver 50 (see FIG. 1). The table represents 64 rows of scramblingvectors, where each row in the table is occupied by a 16-bit scramblingvector. The first scrambling vector of FIG. 2 is shown occupying rownumber “0”. Messages that are longer than 16 bits require the use ofadditional scrambling vectors to continue the scrambling process. Itwould be feasible to devise an algorithm which redirects the pointerafter the completion of the row in which the first scrambling vector islocated. Preferably, the present invention continues the scramblingprocess by automatically incrementing the pointer to the next row (Row1). If the first scrambling vector happens to be in the last row of thetable, the pointer automatically wraps around to select the first row inthe table (Row 0) when the pointer is incremented.

The table can be in a read-only memory in the scrambler/descrambler16/66, or stored in memory that is in communication with thescrambler/descrambler 16/66. Likewise, the table can be generated frommathematical formulae in software or other apparatus that generatespredetermined pseudo-random numbers when the units are turned on, andthen stored. Alternately, the scrambling vectors can be calculated froma formula as they are used. Although the invention is not specificallylimited to a table of 64 rows, 64 rows of 16-bit scrambling vectors areneeded to scramble all the data transmitted in a single time slot in theexemplary OFDM system, using each of the scrambling vectors only once.

Obviously, it is critical that a receiver select the same scramblingvector, or series of vectors, that is used by the transmitter toscramble the original message. Any one of a number of algorithms couldbe devised to direct the table pointers. The exemplary OFDM systemdirects the pointer with an algorithm, which is responsive to theposition of the message in the communication frame structure. Asexplained in the provisional application Ser. No. 60/140,648, the basicunit of transmission in the OFDM system is the TDMA slot, which is 375μs in duration. There are 288 information-bearing OFDM tones in a TDMAslot. These 288 tones are divided into 16 frequency slots (groups) of 18tones each. When the 18 tones in the frequency slot are used forcarrying digital data, 16 of the tones carry actual data using 16-QAMmodulation. The other two tones are used as pilot tones for equalizationpurposes. A single tone modulated using 16-QAM modulation carries fourbits of information. The sixteen information-bearing tones in afrequency slot, therefore, carry a total of 64 bits of information.

The TDMA slots are organized into a framing structure as follows. EightTDMA slots constitute a TDMA frame of 3 ms in duration. A TDMA frame isalso known as a NAC slot. Five NAC slots constitute a NAC frame of 15 msin duration, also known as a BC slot. Thirty-two BC slots make up a BCframe of 480 ms in duration.

The formula used to direct the pointer is as follows:pointer=[(NACSlotNum*16)+BCSlot+(FreqSlotNum*4)]mod64where NACSlotNum refers to the NAC slot number, BCSlot refers to the BCslot number, and FreqSlotNum refers to the frequency channel slotnumber.

Using the above formula, all of the scrambling table is used to scramblethe data in each frequency slot differently. Another advantage of theabove formula is that the scrambling vector used to scramble the data ina given frequency slot changes for each NAC slot and each BC slot. Ifthe scrambling vector used in a first time slot does indeed cause alarge peak in the time domain OFDM signal, causing it to be clipped bythe power amplifier, then the data may not be recovered correctly by thereceiver. In this case, the data is retransmitted in a second timeslot.The second time the data is transmitted, it is scrambled with adifferent scrambling vector, thus reducing the likelihood that a largepeak will be present in the time domain OFDM signal in the secondtransmission.

Obviously, there are many other formulae that could be derived using thesame framing system as described above. Other totally different systemscould be used to direct the pointer, such as systems where the pointeris responsive to a system clock or a system where the base stationdirectly commands the pointer position.

FIG. 4 is a 16-QAM constellation for encoding data using Gray Coding(prior art). The 16-QAM constellation and time domain plots of OFDMwaveforms which follow are presented in order to clarify the advantagesof scrambling digital data patterns.

FIG. 5 illustrates an OFDM time domain waveform of 288 tones with noscrambling. The first 144 tones are selected from constellation point(−3, −3) (see FIG. 4). This constellation point represents the encodingof four binary ones. The other 144 tones are random data that have beenmodulated using 16-QAM. As can be seen in the FIG. 5, a large peak (22dB peak to RMS ratio) is present in the time domain which corresponds tothe pattern of ones.

FIG. 6 illustrates an OFDM time domain waveform of 288 tones withscrambling. Data randomly distributed in the 16-QAM constellation, withno long patterns of ones or zeros, is transmitted. The peak to RMS ratiois only 9.5 dB. A comparison of FIGS. 5 and 6 shows an improvement ofover 10 dB peak to RMS power when the data is scrambled.

FIG. 7 is a flowchart illustrating a method for transmitting an OFDMwaveform at a minimized peak power level. Alternately stated, theflowchart represents a method for minimizing the peak voltage of thetones in an OFDM communication. Although the method is presented as asequence of numbered steps for the purpose of clarity, no order shouldbe inferred from the numbering unless explicitly stated. Step 100 startswith a transmitter in an OFDM communication system. In step 102, thetransmitter accepts a message including digital data. The transmitteraccepts a message which is selected from the group including voicecommunications and high speed data formats. Step 104 randomizes digitaldata patterns in the message, creating a randomized message. Step 106maps the randomized message into OFDM tones. Step 108 is a product wherethe OFDM tones are transmitted at a minimized peak power level.

The OFDM system of step 100 also includes a receiver and in step 110 thereceiver receives the transmitted OFDM tones. Step 112 de-maps thereceived OFDM tones into the randomized message. Step 114 de-randomizesthe pattern of digital data in the message, and step 116 receives themessage.

Preceding the mapping of the randomized message into OFDM tones, step105 modulates the randomized message using a modulation format selectedfrom the group including QPSK, PSK, and QAM modulation formats.Following the de-mapping of the received OFDM tones into the randomizedmessage in step 112, step 113 demodulates the randomized message.

The randomization of the message in step 104 includes combining thedigital data with a predetermined scrambling vector. Likewise, thede-randomization of the message in step 114 includes removing thescrambling vector from the message of digital data. Typically, thescrambling vector is a pseudo-random arrangement of digital data, andthe combination of the digital data with the first scrambling vector instep 104 includes using an exclusive-or process to add the scramblingvector. Likewise, the removal of the scrambling vector from the digitaldata in step 114 includes using an exclusive-or process to add thescrambling vector.

In one aspect of the invention, a plurality of scrambling vectors areprovided. Then, the method comprises further steps. Step 103 selects afirst scrambling vector. Using the first scrambling vector, step 104randomizes the pattern of digital data in the message. Likewise, usingthe first scrambling vector, step 114 de-randomizes the pattern ofdigital data in the message.

In some aspects of the invention, the transmitter includes a table witha first plurality of scrambling vectors arranged in a first plurality ofrows, and the receiver includes a table with the first plurality ofscrambling vectors arranged in a first plurality of rows. Then, theselection of the first scrambling vector for randomizing in step 103includes using a selection algorithm to direct a pointer to a row in thescrambling vector table. Likewise, the selection of the first scramblingvector for de-randomizing in step 114 includes using the selectionalgorithm to direct a pointer to a row in the scrambling vector table.

Typically, the transmitter and the receiver use a common scramblingvector table. In some aspects of the invention step 108 includes theOFDM tones being transmitted in a first position in a frame structure.The selection algorithm used in steps 103 and 114 includes selecting afirst row in the tables in response to the first position of the OFDMtones in the frame structure.

In some aspects of the invention, each row of the table includes ascrambling vector of 16 bits. When the acceptance of the message in step102 includes receiving a message of at least 16 bits, then the selectionof the scrambling vector in step 103 includes the pointer beingautomatically incremented to the next row in the table after the firstrow is used. Likewise, the de-randomization of the message in step 114includes the pointer being automatically incremented to the next row inthe table after the first row is used.

In some aspects of the invention, more than one message is beingtransmitted simultaneously, so that at least two messages require thattheir data be randomized. Then, the acceptance of a message in step 102includes the transmitter accepting a second plurality of messagesincluding digital data. The randomization of the message in step 104includes randomizing the digital data patterns in a second plurality ofmessages. The mapping of the randomized message into OFDM tones in step106 includes mapping the second plurality of randomized messages intoOFDM tones. Likewise, the de-mapping of the received OFDM tones into therandomized message in step 112 includes the receiver de-mapping the OFDMtones into a second plurality of randomized messages. The de-randomizingof the message in step 114 includes de-randomizing the second pluralityof messages, and the receiving of the message in step 116 includesreceiving the second plurality of messages.

The system and method for randomizing digital data patterns describedabove is helpful in reducing degradations due to non-linearity, and forreducing power consumption in power amplifiers. An exemplary OFDMcommunication system was used to illustrate the workings of theinvention, but the invention is not strictly limited to the OFDM systemused in the descriptions. Other embodiments and variations to theinvention will occur to those skilled in the art.

1. A mobile transmitter configured to transmit an orthogonal frequencydivision multiplexed (OFDM) waveform, comprising circuitry configured toaccept a message including digital data, randomize digital data patternsin the message by combining the digital data with a predeterminedscrambling vector to create a randomized message, modulate therandomized message, map the modulated message into OFDM tones, andtransmit the OFDM tones.
 2. A mobile transmitter comprising circuitryconfigured to accept a message including digital data, randomize digitaldata patterns in the message to create a randomized message using datastored in a table of scrambling vectors, map the randomized message intoorthogonal frequency division multiplexed (OFDM) tones, and transmit theOFDM tones.
 3. The transmitter of claim 2, wherein the circuitry isconfigured to randomize by combining the digital data with apredetermined scrambling vector to create the randomized message.
 4. Thetransmitter of claim 3, wherein the scrambling vector is a pseudo-randomarrangement of digital data, and the circuitry is configured to add thedigital data to the scrambling vector using an exclusive-or process. 5.The transmitter of claim 3, wherein the circuitry is configured toselect a first scrambling vector from a plurality of scrambling vectors,and use the first scrambling vector to randomize the pattern of digitaldata in the message.
 6. The transmitter of claim 2, further comprisingthe table of scrambling vectors including a first plurality ofscrambling vectors arranged in a first plurality of rows, wherein thecircuitry is configured to accept the message, and select a firstscrambling vector for randomizing using a selection algorithm to directa pointer to a row in the scrambling vector table.
 7. The transmitter ofclaim 6, wherein each row of the table includes a scrambling vector of16 bits; wherein the message includes at least 16 bits; and wherein thecircuitry is configured to increment the pointer to a next row in thetable, after the row is used.
 8. The transmitter of claim 2, wherein thecircuitry is configured to modulate the randomized message prior tomapping the randomized message into OFDM tones.
 9. The transmitter ofclaim 8, wherein the randomized message is modulated using at least oneof QPSK, QAM, and PSK.
 10. The transmitter of claim 2 wherein themessage is selected from the group including voice communications andhigh speed data.
 11. A receiver to accept transmitted orthogonalfrequency division multiplexed (OFDM) tones, comprising: a de-mapper tode-map received OFDM tones into a modulated message; a demodulator todemodulate the modulated message into a randomized message; and adescrambler to de-randomize the randomized message by removing ascrambling vector from the randomized message.
 12. The receiver of claim11, wherein the descrambler removes the scrambling vector using anexclusive-or process.
 13. The receiver of claim 12, further comprising:a scrambling vector table storing a plurality of scrambling vectors,wherein the receiver employs a selection algorithm to direct a pointerto a first row in the scrambling vector table.
 14. The receiver of claim13, wherein the receiver receives OFDM tones in the first position of aframe structure, and descrambler selection of the scrambling vector isresponsive to the first position of the OFDM tones in the framestructure.
 15. The receiver of claim 13, wherein the scrambling vectortable has rows, wherein each row of the table includes a scramblingvector of 16 bits; and wherein the receiver descrambler automaticallyincrements the pointer to a next row in the table after the first row isused.
 16. A transmitter, comprising: a port to accept a messageincluding digital data; a scrambler to randomize the pattern of digitaldata in the message; a modulator to modulate the randomized message; anda mapper to map the modulated message into orthogonal frequency divisionmultiplexed (OFDM) tones for transmission, wherein the scramblerrandomizes the digital data pattern in the message by combining thedigital data with a predetermined scrambling vector.
 17. The transmitterof claim 16, wherein the scrambler combines the scrambling vector usingan exclusive-or process.
 18. The transmitter of claim 17, furthercomprising: a scrambling vector table including a first plurality ofscrambling vectors organized in a first plurality of rows and whereinthe scrambler employs a selection algorithm to direct a pointer to afirst row in the scrambling vector table.
 19. The transmitter of claim18, wherein the transmitter transmits OFDM tones in a first position ina frame structure, and wherein the base station transmitter scramblerselection of the scrambling vector is responsive to the first positionof the OFDM tones in the frame structure.
 20. The transmitter of claim18, wherein the scrambling vector table has rows, where each row of thetable includes a scrambling vector of 16 bits, and wherein thetransmitter accepts a message including at least 16 bits, and whereinthe scrambler automatically increments the pointer to the next row inthe table after the first row is used.
 21. A receiver, comprising: meansfor receiving transmitted orthogonal frequency division multiplexed(OFDM) tones; means for de-mapping the received OFDM tones into arandomized message that includes a pattern of digital data; means forde-randomizing the pattern of digital data in the randomized messageusing data stored in a table of scrambling vectors; and means forgenerating a message corresponding to the de-randomized pattern ofdigital data.